The Wind Caves at Rock City are in the Domengine formation. The caves are not formed by wind, but in fact are formed by water percolating through the sand and dissolving the cement holding the sand grains together. We also observed iron stained bands in the sandstone known as Liesegang structures. These formed as a source of iron hydroxide diffused through the sandstone creating a beautiful banded appearance in the rock.
Stop 5: Some of the "Wind Caves" in the Domengine sandstone.
Stop 5: Liesegang bands in the Domengine sandstone.
Stop 6 was at Curry Point on South Gate Road. The parking area is in a shaley Early Eocene section resting unconformably on Upper Cretaceous Campanian rocks. Here we viewed the south flank of Mt. Diablo. Much of the south side of Mt. Diablo is composed of greenstone and chert sequences of the Franciscan complex. At the base of this section is the Diablo Thrust that placed the Franciscan complex on Albian shales.
Stop 6: Curry Point - Views towards the summit of Mount Diablo. This is a view of the south flank of Mount Diablo. The valley in the foreground is underlain by Cretaceous shales. Early Eocene shales and sands underlie the parking area. Most of Mount Diablo is composed of Franciscan rocks. The base of this section is the Diablo Thrust. Many landslides bring down Franciscan rocks and obscure the contact.
The Diablo Thrust separates the Tertiary and Campanian sediments in the syncline from the thrusted rocks of Early Cretaceous and Jurassic that are involved in the core of the antiform. In Albian time a section of oceanic crust and its overlying sediments was imbricated in the accretionary prism east of the oceanic trench. In Ron’s interpretation a portion of this oceanic crust slid into a subsea trough and rested on Albian shales deposited in the forearc basin. The Mt. Diablo massif consists of a preserved section of oceanic crust including gabbro/diabase, serpentinite, basalt flows and pillows, and chert.
In driving from Stop 6 to Stop 7 we crossed the Diablo Thrust. Stop 7 was just up the road near The Pines Camp in the Franciscan complex. Here we had an overview of the entire southern flank of the uplift, and back across the San Ramon syncline toward Stop 1 where we began the day. The ridges are formed by two dominant sand sequences. They are the Eocene Domengine sand and the Miocene Briones sand.
Stop 7: A panoramic view to the south and east across Curry Canyon towards Riggs Canyon.
At Stop 8 further up Summit Road we paused to observe some folded and faulted chert beds in the Franciscan. The chert overlies greenstone in other areas not visited on this trip. We continued up Summit Road crossing the axis of the Mount Diablo antiform near Laurel Dell camp.
Stop 8: Along the road to the summit. Folded Franciscan radiolarian cherts.
Stop 8: The group and the cherts.
Stop 8: Folded Franciscan cherts.
Stop 9 was in the parking lot at the summit of Mt. Diablo. The Summit Visitors Center is built from fossil-rich Briones sandstone, but unfortunately is closed now. After taking in the view we proceeded to do a half mile hike around the summit on the Fire Interpretive Trail. Here we could see many of the rocks characteristic of the Franciscan including greenstones, red chert, pillow basalts, graywacke, and argillite.
Stop 9: Summit of Mount Diablo. A view to the north towards Concord and Clayton. The Lone Star Industries quarry on Mt. Zion exposes sheeted diabase dikes of the Coast Range Ophiolite.
Stop 9: A view to the northeast across North Peak.
Stop 9: Franciscan greenstone (pillow basalt) along the "Fire Interpretive Trail" that circumnavigates the summit.
The NCGS sincerely thanks Ron Crane for leading this excellent field trip and providing the field guide describing the trip in detail. We thank Tridib Guha for organizing the trip, handling trip registration, and arranging transportation. We thank Phil Reed and Don Lewis for their continued hard work getting coffee, donuts, lunches, and refreshments. And, of course, we thank all the drivers for the use of their cars.
Diablo Afternoon: An afternoon panoramic view from Stop 1.
The Field Guide: On sale for $5.00 at (most) all meetings!!